The invention relates to thermal devices, such as gasification reactors and boilers, particularly fluidized bed boilers, such as bubbling fluidized bed boilers. The invention relates to thermal devices for heating a heat transfer medium. In particular, the invention relates to thermal devices for heating a heat transfer medium, such as steam, to a very high temperature.
Boilers are used for burning combustible material and thereby for producing energy, such as heat. Heat is recovered from the heat transfer surfaces of the boiler by a heat transfer medium, such as water and/or steam. Hot steam can be used to generate electricity, for example by means of steam turbines.
The efficiency of generating energy is improved when the temperature of the heated heat transfer medium is raised. However, some challenges are involved in increasing the temperature. Increasing the temperature will inevitably increase the temperature of the outer surfaces of the heat transfer pipes. Because corrosive substances, such as salts, are condensed on the surfaces, and an increase in the temperature generally accelerates chemical reactions, corrosion is significantly accelerated due to the increase in the temperature.
Furthermore, for producing particularly hot heat transfer medium, the heat transfer pipe for recovering heat should be placed in a very hot environment. The pressure inside the heat transfer pipe is usually considerable (for example, dozens of bars, typically higher than 30 bar); for example, the pressure and the temperature may correspond to the pressure of saturated vapour, at least at low temperatures. At higher temperatures, the steam is normally superheated, wherein its temperature is higher than the temperature of saturated steam at a corresponding pressure, or the temperature of the critical point of the heat transfer medium, i.e. the critical temperature. (374° C. for water), is exceeded. The heat transfer pipe used in such a hot environment has to withstand the pressure prevailing inside the pipe and also the loads from the corrosive environment outside the pipe. Heat transfer pipes which are resistant to a hot environment and a high pressure under corrosive conditions are typically very expensive options.
Protected heat transfer pipes for a loopseal superheater are disclosed in US 2010/0000474. Therein, the superheating piping includes a steam pipe where the steam to be superheated is directed, and the steam pipe is separated by a protective shell. Such a superheater is preferably placed inside a loopseal. Same principles can be applied for a radiant superheater or a superheater arranged in a flue gas channel.
A device and method for altering the heat transfer characteristics of tubes exposed to the heat generated within a boiler is disclosed in U.S. Pat. No. 4,177,765. Therein a fluidized bed boiler is equipped with a plurality of slidable sleeves circumscribing the vapor generator tubes disposed therein. By selectively extending or retracting the sleeves over the tubes, the heat transfer characteristics can be altered.